When I send an SMS to my twilio phone number I see in the logs:
Error : 11200 HTTP retrieval failure Description: An attempt to retrieve content from https://xxx.xxx.com/extapi/twilio-php/twilio.xml.php returned the HTTP status code .
Here is the content of twilio.xml.php:
<?php
header("content-type: application/xml");
echo '<?xml version="1.0" encoding="UTF-8"?>';
?>
<Response>
<Sms>test</Sms>
</Response>
I found the solution. I dissable SSL Certificate Validation in Twilio's account settings.